1. Field of the Invention
The present invention is directed to a positioning method and apparatus for defining the relative positions of a first object and a second object.
2. Description of the Related Art
The relative positions of a wire electrode and a workpiece of an electrical discharge machine (EDM) for performing electrical discharge machining must be defined accurately prior to the start of machining in order to ensure high machining accuracy.
To define the positional relationship, it is known in the art to provide the EDM with a contact position setting function which brings the electrode and the workpiece into direct contact and sets that contact position as a reference. The reference is used by a numerical control (NC) unit to control the positioning of the electrode and the workpiece relative to each other.
The prior art wire-cut EDM described above is shown in FIG. 5. The positioning of wire electrode 1 relative to the workpiece 2 is controlled by NC unit 15. More specifically, the workpiece 2 is positioned by moving table 3 in either the X or the Y direction, using corresponding motors 5 and 6. The wire electrode 1 is fed through the EDM by use of main tension rollers 7 and bottom rollers 8. A contact detector 14 detects the contact of the wire electrode 1 with the workpiece 2 and outputs the detection signal to NC unit 15.
An example of a typical contact detector 14 used in the prior art EDM is shown in FIG. 6. The detector 14 receives inputs from wire electrode 1 and the workpiece 2. A voltage source 10 supplies a contact detecting voltage to the machining gap. A comparator 11 compares the voltage level in the machining gap to determine whether there is contact between the wire electrode and the workpiece 2. The comparator 11 outputs a signal to the NC unit 15, indicating that contact has been determined (i.e., voltage V=V.sub.b). A current limiting resistor 13 is used to limit the contact detecting voltage across the machining gap. A switch 12 can be used by the contact detector 14 to sample the gap voltage V alternately.
In operation, the NC unit 15 receives a command requesting a contact position setting operation to be performed. At this time, the contact detector 14 closes switch 12 and applies a voltage from source 10 across the machining gap via the current limiting resistor 13. The wire electrode 1 and the workpiece 2 begin approaching each other in accordance with commands output by the NC unit 15. During the operation, the gap voltage V is compared with a predetermined reference voltage V.sub.b with the comparator 11 to determine whether the wire electrode 1 and the workpiece 2 have made contact. When the gap voltage V becomes lower than the reference voltage V.sub.b, an output signal is transmitted by the comparator 11 to the NC unit 15, as described above. At this point, the NC unit 15 causes the wire electrode 1 and the workpiece 2 to stop their approach, and the contact position setting operation is complete. The relationship between the machining gap distance and the gap voltage during the above-described contact position setting operation is shown in FIG. 7.
The gap voltage V can be represented by the following expression: EQU V=E-Ri (1)
where E represents the output voltage of the voltage supply unit 10, R represents the resistance value of the current limiting resistor 13, and i represents the value of the current flowing from the voltage supply unit 10 to the machining gap via the current limiting resistor 13. Thus, it can be seen that the value of the current i is restricted by the resistance of the wire electrode 1 and the resistance of the machining gap. Assuming that the resistance of the wire electrode 1 is R1 and that of the machining gap is R2, the current i can be represented by: EQU i =E/(R+R1+R2) (2)
It should be noted that the resistance of the machining gap R2 reduces in proportion to the decrease in the gap distance. Hence, as the gap distance reduces, the current i increases and the gap voltage V decreases. The wire resistance R1 reduces in proportion to the increase in the diameter of the wire electrode 1.
For example, when a wire electrode 1 with a large diameter is to be machined with a workpiece 2 having a large thickness, the wire resistance Rl is small and the gap R2 decreases to a great degree as the wire electrode 1 and the workpiece approach each other (i.e., the machining gap distance decreases). In this case, the current i flowing in the machining gap increases when the wire electrode 1 and the workpiece 2 have not yet made contact, and the gap voltage V falls considerably as compared to the open voltage E, as shown in FIG. 8. In contrast, when a wire electrode 1 with a small diameter is used with a workpiece having a small thickness, the wire resistance R1 is large, and the gap resistance R2 decreases to a lesser degree as the wire electrode 1 and the workpiece 2 approach each other, hence the current i increases slightly and the gap voltage V falls slightly. When the wire electrode 1 and the workpiece 2 have made contact, the resistance of the wire electrode Rl is large and the area of contact is small as compared to the positioning of the wire electrode 1 with the large diameter and the workpiece 2 having a large thickness. The current i, then, and the gap voltage V are only slightly reduced.
As the reference voltage V.sub.b of the comparator 11 is fixed, the accuracy of the positioning in the prior art EDM varies according to the types of wire electrodes and workpieces used. For instance, if the contact voltage V.sub.2 has been set to approximate the reference voltage V.sub.b, a contact is detected considerably ahead of the contact position of the wire electrode 1 and the workpiece 2 during the positioning of a wire electrode 1 having a large diameter and a workpiece 2 having a large thickness. This is because the reference voltage V.sub.b (set approximately equal to V2) is higher than the actual contact voltage V.sub.1, as shown in FIG. 8.
Similarly, if the contact voltage V.sub.1 has been set to approximate the reference voltage V.sub.b, a contact cannot be detected during the positioning of the wire electrode 1 having a small diameter and the workpiece 2 having a small thickness. In this case, the gap voltage V does not fall below the reference voltage V.sub.b due to the resistance R1 of the wire electrode 1. Hence, if the wire electrode 1 having a small diameter is positioned to contact the workpiece 2 having a small thickness, the reference voltage V.sub.b must be set to a value higher than the contact voltage V.sub.1 in order for the actual contact to be detected accurately.
In addition, the prior art EDM may be exposed to galvanic corrosion, as shown in FIG. 9, caused by a large current flow in the machining gap. Commonly, in the final stages of the above-described positioning function the gap resistance reduces to an extremely small value as the gap distance approaches 0. As a result, large current may flow in the machining gap causing galvanic corrosion.
Moreover, where the actual contact could not be detected accurately, the workpiece 2 would make contact with a moving wire electrode 1 causing the wire electrode 1 to fray or create shavings, as shown in FIG. 10. Shayings 16 within the machining gap tend to make positioning impossible.